With respect to sewing machines which form lock stitches, there have been made proposals for setting concatenating or crossing points of an upper thread and a lower thread in the thickness of a fabric to be sewn by adjusting the tension or the supplying amount of the upper thread.
The stitches to be formed in the fabric generally include straight stitches and various types of zizag stitches, the former being produced by a vertically reciprocating needle while the fabric is transported relative to the needle, and the latter being formed by adding thereto lateral movements of the needle. The upper thread tension and the amount of the supplied upper thread is adjusted in response to the required amount of the upper thread in these types of stitches.
The straight stitches and the zigzag stitches are generally required to have different positions of proper crossing points of the upper and lower threads within the thickness of the fabric. The former is required to have the crossing point set at about the center in the fabric thickness. The allowance is that the crossing point does not appear on the upper surface or the rear surface of the fabric. The thinner is the fabric, the narrower is the allowance, and the more exact adjustment is required, accordingly. Since the required amount of the upper thread is determined by the feeding amount and the thickness of the fabric, it does not vary per each stitching.
The zizag stitches are generally required to have the crossing points set at the rear surface of the fabric. Because if the crossing point is set at the center of the fabric, the lower thread is seen from the upper surface of the fabric through needle perforations of the fabric, and if the both threads have different colors, the lower thread must be prevented from mixing into patterns formed with the upper thread only.
Therefore, the crossing point is set at the rear side of the fabric, and the lower thread is to extend within a range of 1/4 to 3/4 of a set zigzag width as shown in FIG. 3. In order to cover the remaining range with the upper thread, adjustment of the upper thread tension and the supplying amount of the upper thread is required. The adjusting allowance is larger than that of the straight stitches. The zigzag stitch patterns will include stitches of different amplitude.
When adjusting the tension of the upper thread, a fine adjustment per each of stitches is difficult but it is possible to set the tension at a constant value.
On the other hand, when adjusting the supplying amount of the upper thread, the fine adjustment per each stitch is easy, but in this case, control data is required per each stitch, and a control device is therefore complicated.